D2D communication is a kind of communication which allows two user equipments to communicate with each other directly, potentially under some control of cellular networks. The two user equipments may be referred to as a D2D pair. The examples of D2D communication include direct communications in a cluster of proximity devices, autonomous D2D communication in a cellular network, etc.
Nowadays, many investigations and researches about supports of network-controlled D2D communication in 3G or 4G radio communication networks, e.g. Long Term Evolution (LTE) network or LTE-Advanced network, have been conducted.
For the D2D communication, D2D Random Access Channel (RACH) is introduced. The radio resources for the D2D RACH can be allocated by a network-side device such as an eNodeB. The D2D RACH would be quite similar to the LTE Physical Uplink Control Channel (PUCCH). One user equipment may utilize the D2D RACH to access the other user equipment directly. Over the D2D RACH, a D2D connection can be established between two user equipments, and the data can be transmitted over the D2D connection.
During the data transmission of the D2D pair, there may be a radio link failure (RLF) on the D2D radio link due to the complexity of heterogeneous network. The D2D radio link tends to suffer from the heavy interference from other D2D pairs or cellular user equipments (assuming cellular uplink band can be also used for D2D communications), so the RLF will be unavoidable.
Generally, after the RLF occurs on the radio link, a connection re-establishment will be initiated to recover the connection. However, the connection re-establishment for a D2D connection is different from the connection re-establishment for a radio resource control (RRC) connection in a cellular network such as LTE network.
In the conventional RRC connection re-establishment procedure, after the RLF or handover failure, the user equipment firstly will select a cell for re-establishment, and then sends a RRC Connection Reestablishment Request to the eNodeB. Then the eNodeB will do verification, and upon successful verification, allocate radio resources for the re-establishment and send a RRC Connection Reestablishment message to the user equipment to update related parameters of signaling radio bearers. Finally after all the parameters are updated successfully, the user equipment will send a RRC Connection Reestablishment Complete message to the eNodeB. So the RRC connection re-establishment procedure happens between the user equipment and the eNodeB, and the verification is done by the eNodeB.
But such the RRC connection re-establishment procedure cannot be applied to the D2D connection re-establishment, because the D2D connection re-establishment happens between the two user equipments.
When the RLF occurs on the D2D radio link, in order to resume the D2D connection, a new connection setup procedure may be initiated for the D2D radio link. But this will cause data loss and possibly involve core network due to generation of a new security key for the D2D communication. This kind of D2D connection re-establishment will have a negative impact on user experience for the D2D communication and also push too much signaling burden to the core network side.